HELLO
The Hello World Program, OpenMP version

HELLO is a simple program that says "Hello", but also demonstrates a few simple things about OpenMP programs, including

• How you get the "include" file.
• How you measure wall clock time.
• How you find out how many processors are available.
• How you find out which thread you are in a parallel section.
• How you find out how many threads are available in a parallel section.
• How you set the number of threads.

To use the program, you will also need to figure out the appropriate compiler switches to use, and how to set the enviroment variable OMP_NUM_THREADS to the number of threads you want.

Assignment:

Take this program to the machine you are studying. Compile it with the appropriate compiler switch.

• Run the program without setting the OMP_NUM_THREADS environment variable. You will get the default number of threads, which should be 1.
• Rerun the program, but now define the environment variable so that you request 4 threads.
• Modify the program so that it ignores the environment variable, and instead always requests the number of threads that is equal to the number of processors. Run the program and verify that your change works - that is, set the environment variable to something ``silly'', and demonstrate that the program ignores you.

If you are actually running on a multiprocessor machine, you may notice that the output does not always come out in a nice order. It can be somewhat scrambled. The output from a particular thread will be ordered, but output from different threads can be interleaved in somewhat arbitrary ways - this is similar to the way that the program instructions, when carried out by multiple threads, can be interleaved in ways you would not imagine.

Source Code:

To run the HELLO program, you first need to copy one of these source code files, compile it using the appropriate compiler switches, and be sure to name the executable hello.

• hello.c, the C source code.
• hello.cc, the C++ source code.
• hello.f, the FORTRAN77 source code.
• hello.f90, the FORTRAN90 source code.

Batch Script:

Once you have made a hello executable, you need to copy this script file to the same directory that contains the executable, and issue the command qsub hello.sh. That submits a request to have the executable program run on 4 processors. You can type showq to see if the request has started yet.

• hello.sh, the job submission script.

You can go up one level to the HPPC 2008 web page.